A Bay Area federal judge says the Obama administration can keep secret a memo spelling out the legal rationale for a 2011 drone attack in Yemen that killed a U.S. citizen and alleged terrorist mastermind.

The Justice Department was entitled to withhold the memo on the grounds of national security and lawyer-client confidentiality, Chief U.S. District Judge Claudia Wilken of Oakland said Friday.

Although U.S. Attorney General Eric Holder and other Obama administration officials have made public statements justifying drone strikes, Wilken said none of them was specific enough for her to rule against the government's claim of secrecy and require officials to disclose the legal rationale for the Yemen attack.

Sued by local group

The ruling dismissed a suit by the First Amendment Coalition, an open-government advocacy group in San Rafael. The organization sued after a September 2011 drone strike in Yemen that killed Anwar al-Awlaki, a U.S.-born Muslim cleric whom authorities suspected of organizing an attempt to blow up a Detroit-bound airliner in 2009. Another U.S. citizen was also killed in the drone attack, and Awlaki's U.S.-born, 16-year-old son was killed by a drone in Yemen the following month.

On April 4, a judge in Washington, D.C., dismissed a damage suit against U.S. officials by parents of those killed in the drone strikes, saying officials who order such attacks "must be trusted" to act legally and are not subject to judicial oversight.

In its lawsuit, the First Amendment Coalition said it wasn't seeking disclosure of classified intelligence information, just the legal rationale for killing U.S. citizens without a trial or judicial review.

That rationale is reportedly described in a 16-page Defense Department "white paper" that was leaked to the news media last year after being circulated to members of Congress. Among its conclusions was that the U.S. can launch a drone attack when it suspects the target is planning violence and does not have to wait until it has evidence of an immediate assault.

The First Amendment Coalition's lawyers argued that the government waived any claim of confidentiality when Holder, in a March 2012 speech, outlined the "legal principles" for such attacks - that a U.S. citizen must post an imminent threat of violence against the United States, and capture is "not feasible."

Other administration legal officials have publicly discussed similar standards for lethal strikes without prior judicial approval.

But Wilken said the officials had only described overall legal principles that applied to targeted killings and had not spelled out the reasoning that apparently is contained in the Defense Department memo. The government is entitled to withhold the "confidential legal advice" its lawyers provided on the sensitive subject, the judge said.

Thomas Burke, a lawyer for the First Amendment Coalition, said he expects to appeal the ruling.

"The administration claims and insists it is being transparent about this program," Burke said, but "all that is happening now is the proverbial 'Trust us.' "